The invention relates to a switching power supply unit having an improved power supply efficiency when operating under a light load.
A switching power supply unit controls electric power that provides to a load by turning on and off a switch (e.g. power transistor) connected in series or parallel with the load. Such switching power supply unit uses a smoothing coil and an output condenser in order to stabilize a load current.
The duty factor of the switch controlling the supply of electric power to the load is changed by, for example, regulating the period (or pulse width) of the on-state pulses while fixing the frequency of the pulses.
Switching power supply units are in general more efficient than series power supply units. However, the efficiency of a switching power supply unit depends on the magnitude of the output current. The efficiency goes down with the output current for a small or light load. The lowering of the efficiency is mainly due to the power loss accompanying the switching operation of the power supply unit. The loss is called switching loss.
Therefore, in order to reduce the switching loss of a switching power supply unit under a light load, the unit is harnessed by a burst mode control or a skip mode control.
In the burst mode control, when the output current is detected to be sufficiently small, the power is cut off over a period while an output condenser can substantially sustain a predetermined output voltage, as taught in Japanese Patent Early Publication H-06-303766. That is, on-off switching periods in which power bursts and off-periods alternate. During the off-periods, no switching loss takes place, so that the efficiency is improved accordingly.
In a skip-mode control, a window having a fixed voltage width is applied to the output voltage by a window comparator. In this case, the power supply unit is switched with pulses having a fixed duty factor while raising the output voltage from the lower limit to the upper limit of the window. However, the output voltage is left freely falling from the upper limit to the lower limit. The efficiency is improved while the output voltage is falling from the upper limit to the lower limit, since no switching loss is involved then.
However, under a burst-mode control, burst noises are generated in such conventional switching power supply unit as the unit undergoes a transition from an on-off period to an off-period due to the fact that the switching frequency changes discontinuously in the transition. The burst noise gives adverse effects to the surrounding electronic devices, and can result in mutual interference among them.
A switching power supply unit also results in bursts of output power in a skip-mode control because on-off switching is performed only when the output voltage is raised from the lower limit to the upper limit. Besides, large ripples result in the varying output voltage. Such large ripples in the output voltage are unfavorable for the load.
It is, therefore, an object of the invention to provide an improved switching power supply unit capable of holding the switching frequency substantially constant at a low frequency to prevent the switching frequency from discontinuously changing and to suppress ripples in the output voltage when the load is light, thereby resulting in only a limited power loss for the light load.
In accordance with one aspect of the invention, there is provided a switching power supply unit, comprising:
a switching circuit for switching on and off an input voltage in response to an instruction signal received;
a smoothing coil connected to the output end of the switching circuit for smoothing the switching output of the switching circuit;
an output condenser connected to the output end of the smoothing coil and charged to the output voltage of the power supply unit,
a pulse signal generator for generating a periodic pulse signal having a duty factor in accord with the difference between a reference voltage and a feedback voltage associated with the output voltage; and
a delay-control unit receiving the pulse signal from the pulse signal generator and a light-load determination signal indicating whether the load connected to the switching power supply unit is light or not to output as the pulse signal the instruction signal as it is when the load of the power supply unit is not light, but otherwise output the pulse signal after widening the pulse width thereof.
In accordance with another aspect of the invention, there is provided a switching power supply unit, comprising:
a switching circuit for switching on and off an input voltage in response to an instruction signal received;
a smoothing coil connected to the output end of the switching circuit for smoothing the switching output of the switching circuit;
an output condenser connected to the output end of the smoothing coil and charged to the output voltage of the power supply unit,
a pulse signal generator for generating a first periodic pulse signal having a duty factor in accord with the difference between a reference voltage and the feedback voltage representing the output voltage; and
a signal-thinning control unit receiving the first periodic pulse signal from the pulse signal generator and a light-load determination signal indicating whether the load connected to the switching power supply unit is light or not to output as the instruction signal the first periodic pulse signal when the load of the power supply unit is not light, but otherwise output a second periodic pulse signal that are generated by thinning the first periodic pulse signal.
With a switching power supply unit of the invention, the switching frequency of the unit is held substantially constant at a low frequency when the load is light, thereby reducing the power loss and improving the efficiency of the unit during a light-load period. In addition, burst noise substantially disappear, and hence ripple of the output voltage, may be reduced negligibly small as compared with conventional ones.